The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for combined magnetic resonance elastography and chemical-shift encoded imaging.
A diseased organ, such as the liver or heart, will often exhibit increases in one or more of the following: stiffness, as in fibrosis or cirrhosis; fat content, water content, or both, as in steatosis; and iron content, as in hemochromatosis. MRI can be sensitized to, and can quantitatively assess, each of the aforementioned physiological properties.
Having access to a wide variety of information improves a physician's ability to differentially diagnose complex disease. For many applications, a typical MRI protocol entails performing, sequentially, several different scan types, each aimed at revealing a different type of physiological information. For example, when studying non-alcoholic fatty liver disease (“NAFLD”), both a chemical shift encoded sequence and a magnetic resonance elastography (“MRE”) sequence are commonly performed to provide a battery of complementary information. The chemical shift encoded sequence provides information useful for assessing fat fraction and iron content, whereas the MRE sequence provides information useful for assessing tissue stiffness.
NAFLD is a condition that is increasing in prevalence in western countries and can lead to liver fibrosis and end-stage liver disease. In the past, the only option for clinicians to obtain quantitative information about the extent of steatosis and fibrosis was to perform a liver biopsy. MRI-based techniques are increasingly being used as safer, more comfortable, and less expensive alternatives to biopsy in this assessment.
Chemical shift encoded sequences, such as Dixon techniques with advanced processing, have been shown to provide accurate quantitative estimates of proton-density fat fraction throughout the entire liver in very short acquisition times. Similarly, MRE has been shown to provide reliable quantitative assessment of liver fibrosis throughout the entire liver in scan times as short as one minute.
In currently existing multiparametic imaging techniques, two or more separate scans are required to collect data: one for each physiological parameter for which information is desired. Different quantities are then extracted from each of these separate data sets. For instance, one acquisition would be required to obtain water-fat information and a separate acquisition would be required to obtain stiffness information. These existing approaches are straightforward in their implementation, but are limited in their clinical utility because they require prolonged scan time and often exhibit suboptimal imaging signal-to-noise ratio (“SNR”). These existing approaches are also at a disadvantage in that the multiple different images must be co-registered to allow direct comparisons, and misregistrations resulting from subject motion are commonplace. Physical effects are also not decoupled in the acquired data sets.